Apparatus for focussing the beam of a particle accelerator

ABSTRACT

Apparatus for focussing the beam of a particle accelerator. A resistive chain is connected across the high voltage source for the accelerator. This chain includes, inter alia, a potentiometer chain for supplying varying potentials to the accelerating electrodes on a pair of fixed resistors for supplying potential to the anode of the electron gun. The beam injection system includes two pairs of deflection plates which receive potential from two ganged pairs of potentiometers wired in parallel across the pair of fixed resistors.

United States Patent [111 3,810,030 Adler May 7, 1974 [54] APPARATUS FOR FOCUSSING THE BEAM 3,117,230 1/1964 Yule 313/63 x 017 A PARTICLE ACCELERATOR 3,375,390 3/l968 Schlesinger 313/83 3,448,316 6/1969 Yoshida ct al 313/70 C X [75] Inventor: Hermann Adler, Remach,

Switzerland Prima Examiner-Palmer C. Demeo E 1H t I & C AG B l [73] Asslgnee x1 55 3 le as e Attorney, Agent, or Firm-Kurt Kelman [22] Filed: May 20, 1971 [21] Appl. No.: 145,460 [57] ABSTRACT Foreign ApPucation Priority Data Apparatus for focussing the beam of a particle accel- May 21, 1970 Switzerland 7528/70 erator. A resistive chain is connected across the high voltage source for the accelerator. This chain in- [52] US. Cl 328/228, 313/63, 328/229, cludes, inter alia, a potentiometer chain for supplying 328/233, 328/267 varying potentials to the accelerating electrodes on a [51] Int. Cl. H0lj 29/74 pair of fixed resistors for supplying potential to the Field of Search 328/228, 233, 267, 229; anode of the electron gun. The beam injection system 313/63 includes two pairs of deflection plates which receive potential from two ganged pairs of otentiometers [5 6] References Cited wired in parallel across the pair of fixed resistors.

UNITED STATES PATENTS 3,602,827 8/1971 Peoples et a] 328/233 1 Claim, 2 Drawing Figures ll 4 5 r-Z: l4 5 J is f :L! fl L ZI l I 1 7.2 |2.2- L f 5] a a 7 B A i I 1 i I l l l |2n L/ I SHEET 2 0F 2 PATENTEDIAY 7 014 r INVENTOR. HERMAN ADLER FIG.2

APPARATUS FOR FOCUSSING THE BEAM OF A PARTICLE ACCELERATOR In the construction of particle accelerators in the lower energy region (several MeV), where one is working with straight accelerator tubes and static electric fields, the power supply to the particle source plays a not unimportantrole. In most cases the source is at the same accelerator potential so that the power for its operation must be raised to this potential by means such as an insulating transformer or insulating generator. Also the power available must be converted to the finally required form which requires the necessary equipment. This equipment can be fairly complicated if it must, for example, be controlled or very stable, and, as a rule, it contains semiconductor elements. Since it has been found by experience that it can be very difficult to give complete protection to semiconductors which are sensitive to over-voltage during flash-over of the accelerator voltage, feeding equipment provides certain problems in this respect also.

When it comes to an accelerator with pressurised gas insulation, spacerequirements play an important role, since the weightand the costs for the pressure tank and the filling gas riserapidly with increasing tank size. For this reason on grounds of reliability of operation, space-economy and construction costs, it is a great advantage if one can dispense with an energy source and feeding equipment which are at accelerator potential. Especially in electron accelerators the possibility arises, on account of the simplicity of an electron source, of feeding the latter in large measure with power obtained from the high-voltage generator, whilst as many voltages as possible are taken from a control-potentiometer. This control potentiometer is connected up in series with the potentiometer normally used forthe control of the accelerator tube. To be sure the heater supply must, in so far as the electron-gain possesses a heated cathode, be converted in another way to the high potential.

The equipment according to the invention is advantageously used where the electron beam supplied by the electron guniis used or where the beam current is small compared with the current of the control potentiometer of the accelerator tube.

Equipment according to the invention will now be described, by way of example, with reference to the two embodiments represented in the accompanying drawings.

FIG. 1 is a schematic and sectional view of a first particle accelerator embodiment according to this invention;

FIG. 2 is a schematic and sectional view of a second particle accelerator embodiment according to this invention.

Referring to FIG. 1, the numeral 1 denotes an injection system in the form of an electron gun consisting of a cathode 2, a modulator electrode 3 and an anode 4, in which the modulator. electrode is at the same potential as the cathode and merely serves as its high-voltage shield. The injection system 1 is mounted on an accelerator tube 5. The anode voltage of the electron gun is taken from a resistor 6 which is connected in series with a control potentiometer 7 arranged on the electron tube, the control potentiometer consisting of the resistor portions 7.1 7.n whose terminals are connected with the accelerator electrodes 12 12n. A high voltage generator is designated by 8, its voltage being developed between point 9 and earth.

Changing the resistance of the resistor 6 causes a change in the energy of entry of the electrons into the accelerator tube and hence a change in the focal length of the tube. The image 10 of the constriction 11 of the beam can thus be adjusted inside the anode. The resistor 6 thus serves as the focusing element of the control equipment.

FIG. 2 shows a further embodiment. The various components or groups are represented in the two embodiments by the same reference numerals. The injection system 1 is provided with a beam-refractor which displays two plate-pairs 14 of which for representational reasons only one pair is shown. The second pair of plates is situated in the same plane as and coaxially with the pair shown, but at right angles to the latter. The electron gun is in this embodiment operated as a triode, since the potential of the modulator electrode 3 is separate from that of the cathode 2. The resistor 6 of FIG. 1 is replaced by a resistance arrangement consisting of the resistors 6.1 6.8. Their function can be seen from the following description.

The potential of the modulator electrode 3 is obtained across the resistor 6.1. The voltage between the cathode and the modulator electrode is the algebraic sum of the voltage drop across the resistor 6.1 and of the voltage across a resistor 13 lying between the junction point 9 and the cathode supply-lead. By means of these two resistors the emission beam can be varied.

The resistor 13 also has a stabilising action, since the dependence of the beam current on the geometry of the electron gun and on the cathode temperature is reduced by increasing the resistance of the resistor. This stabilising action is especially to be desired when the beam energy must be highly stable. In this case a good constancy of the beam current is indispensable, since the beam energy resulting from the voltage drop across the resistor 13 is lower than the voltage of the generator 8, which (voltage) frequently in such cases represents the stabilised value.

The resistors 6.3 6.8 supply a voltage for the two pairs of deflection plates referred to, which voltage is symmetrical to the anode potential, which owing to restricted space is. represented by the broken line 16 below the electrode 15 which is to be counted as part of the injection system 1.

The resistors 6.3 and 6.4 which are connected in series have their common point connected with the anode, and are equal in value. The resistors 6.5 and 6.6 as well as the resistors 6.7 and 6.8 are provided in two double rotary potentiometers which may be operated arately. The first resistor pair 6.5/6.6 supplies the deflection voltage to the illustrated pair of plates 14 while the second resistor pair 6.7/6.8 suppliesthe deflection voltage, via leads 17, to the non-illustrated second pair of deflection plates. The double potentiometers are so constructed that one of "the two potentiometers, which are coupled together for the purpose of synchronous running of the tap, is reverse parallel connected relative to the other. The advantage of accomplishing the electrostatic deflection in this manner is that the deflection angle given by the position of the double potentiometer is largely independent of the accelerator voltage.

The resistor 6.2 must possess a certain variability. If the accelerator voltage has to be varied by a significant amount, an adjustment of the resistance of resistor 6.2 is necessary to correct for the axial displacement of the constriction ll of the beam resulting therefrom below the anode and the relativistic change of mass of the electrons, so that the beam constriction 10 below the accelerator tube remains in the same place.

From a voltage standpoint, the division between the beam-injection system and the accelerator-tube 5 logically occurs at the lower potential surface of the anode represented by the broken line 16. From this viewpoint the resistor 6.4 and that portion of the resistors 6.5 6.8 which in FIG. 2 correspond to the lower half of the symbol indicating the resistors ought to have formed part of the control potentiometer 7 of the accelerator tube 5, which however is not shown in FIG. 2 for reasons of clarity.

What is claimed is:

l. in a particle accelerator of the type that includes:

a cylindrical accelerator tube having a plurality of accelerator electrodes disposed therealong;

an injection system mounted to said accelerator tube for supplying a beam of electrons thereto, said system including a cathode, a modulating electrode and an anode; and

a source of high potential for establishing a potential gradient between said cathode and said accelerator electrodes,

the improvement which comprises at least one pair of deflection plates mounted within said injection system; and

a resistive chain connected across said high potential source for supplying different potentials to the several component electrodes of said particle accelerator, said chain comprising, in series connection:

a potentiometer comprising a chain of resistors, the junctures between adjacent resistors in said chain being connected to corresponding ones of said plurality of accelerator electrodes to supply potential thereto from said source;

a pair of substantially identical resistors in series connection, the juncture of said resistors being connected to said anode to supply potential thereto from said source;

at least one pair of mechanically ganged potentiometers connected in parallel with the series connection of said pair of identical resistors, the sliding contact on each potentiometer in the pair being connected to respective ones of said pair of deflection plates, one potentiometer in said pair being inversely wired with respect to the other; and

first and second serially connected variable resistors, the juncture between said variable resistors being connected to said modulator electrode whereby adjustment of said first variable resistor alters the potential of said modulator electrode with respect to said cathode, and adjustment of said second variable resistor alters the potential of both said cathode and said modulator with respect to said anode; and

a third variable resistor serially connected between the juncture of said resistive chain which is proximate said first variable resistor and one terminal of said source and said cathode to stabilize the beam current from said injection system. 

1. In a particle accelerator of the type that includes: a cylindrical accelerator tube having a plurality of accelerator electrodes disposed therealong; an injection system mounted to said accelerator tube for supplying a beam of electrons thereto, said system including a cathode, a modulating electrode and an anode; and a source of high potential for establishing a potential gradient between said cathode and said accelerator electrodes, the improvement which comprises at least one pair of deflection plates mounted within said injection system; and a resistive chain connected across said high potential source for supplying different potentials to the several component electrodes of said particle accelerator, said chain comprising, in series connection: a potentiometer comprising a chain of resistors, the junctures between adjacent resistors in said chain being connected to corresponding ones of said plurality of accelerator electrodes to supply potential thereto from said source; a pair of substantially identical resistors in series connection, the juncture of said resistors being connected to said anode to supply potential thereto from said source; at least one pair of mechanically ganged potentiometers connected in parallel with the series connection of said pair of identical resistors, the sliding contact on each potentiometer in the pair being connected to respective ones of said pair of deflection plates, one potentiometer in said pair being inversely wired with respect to the other; and first and second serially connected variable resistors, the juncture between said variable resistors being connected to said modulator electrode whereby adjustment of said first variable resistor alters the potential of said modulator electrode with respect to said cathode, and adjustment of said second variable resistor alters the potential of both said cathode and said modulator with respect to said anode; and a third variable resistor serially connected between the juncture of said resistive chain which is proximate said first variable resistor and one terminal of said source and said cathode to stabilize the beam current from said injection system. 